The present invention relates to a tuned oscillator used, for example, a local oscillator in microwave communication equipments.
There has been a proposal of a tuned oscillator using ferromagnetic resonance of yttrium iron garnet (YIG) thin film as shown in U.S. Pat. No. 4,626,800 assigned to the present assignee.
This YIG thin film tuned oscillator consists of an active element for oscillation, an YIG thin film resonator, a magnetic circuit for applying a D.C. magnetic field to the YIG thin film in the direction normal to the film surface, and an impedance matching circuit. The YIG thin film tuned oscillator has the oscillation condition expressed in terms of the reflection coefficient Fa seen from the reference plane between the active element and the impedance matching circuit toward the active element and the reflection coefficient .GAMMA.l seen from the reference plane toward the load, as follows. EQU .GAMMA.a..GAMMA.l.gtoreq. (1)
The equality is established for the steady-state oscillation.
Conventionally, the impedance matching circuit has been adjusted so that a maximal fundamental wave output and minimal phase noise at the load are achieved, while retaining the condition of formula (1).
However, the ferrimagnetic resonant frequency of YIG thin film is proportional to the D.C. magnetic field applied to the YIG thin film, and if the tuned oscillator is designed to produce a high-frequency oscillation output, e.g., above 10 GHz, it is necessary to generate a strong D.C. magnetic field, resulting in an increased load to the magnetic circuit. A conceivable counter measure is to minimize the gap length in the magnetic circuit, however, a too small gap length causes adverse effects on the characteristics of YIG thin film resonator and also makes it difficult to assemble in a package.